CT and MR imaging features of mixed neuroendocrine–non-neuroendocrine neoplasm of the pancreas compared with pancreatic ductal adenocarcinoma and neuroendocrine tumor

Objective This study aimed to assess the computed tomography (CT) and magnetic resonance imaging (MRI) features of pancreatic mixed neuroendocrine–non-neuroendocrine neoplasm (MiNEN) and compare them with those of pancreatic ductal adenocarcinoma (PDAC) and neuroendocrine tumor (NET). Methods Twelve patients with pancreatic MiNEN, 24 patients with PDAC, and 24 patients with NET, who underwent both contrast-enhanced CT and MRI, were included. Clinical data and the key imaging features were retrospectively evaluated by two independent readers and compared between MiNEN and PDAC or NET. Univariate and multivariable logistic regression analyses were performed to obtain predictors for pancreatic MiNEN. Results Patients with pancreatic MiNEN more frequently presented with large size and heterogeneous and cystic components compared with PDAC (p < 0.031) and ill-defined irregular margins, progressive enhancement, and adjacent organ involvement compared with NET (p < 0.036). However, vascular invasion was less commonly seen in MiNEN than PDAC (p = 0.010). Moderate enhancement was observed more frequently in MiNEN than in PDAC or NET (p < 0.001). Multivariate logistic analyses demonstrated that moderate enhancement and ill-defined irregular margin were the most valuable features for the prediction of pancreatic MiNEN (p ≤ 0.044). The combination of the two features resulted in a specificity of 93.8%, sensitivity of 83.3%, and accuracy of 91.7%. Conclusions We have mainly described the radiological findings of pancreatic MiNEN with ill-defined irregular margin and moderate enhancement compared with PDAC and NET. The combination of imaging features could improve diagnostic efficiency and help in the selection of the correct treatment method.


Introduction
Mixed neuroendocrine-non-neuroendocrine neoplasm (MiNEN) is commonly found in the stomach [1,2], colon [3,4], and nasopharynx [5]. According to the World Health Organization classification, MiNEN is defined as a heterogeneous tumor with two morphologically different neoplastic components, which include one neuroendocrine, and each component exceeds 30% of the tumor [6,7]. The neuroendocrine part is always present [8]. As a very rare disease, it accounts for only 0.2% of all tumors in the pancreas [9].
Pancreatic MiNEN is a highly malignant tumor that exhibits variable clinical manifestations and poor prognosis with a five-year overall survival rate of 40% [10,11]. The radiological and clinical features of MiNEN resemble those of neuroendocrine tumor (NET) or pancreatic ductal adenocarcinoma (PDAC), but the treatment for these three diseases differ, making differential diagnosis helpful to opt suitable treatment strategies for patients with pancreatic MiNEN [12].
Knowledge on MiNEN of the pancreas is limited because of the rarity of the disease and the lack of comprehensive studies. Previous studies have mainly focused on the clinicopathological features of pancreatic MiNEN and much less on radiological findings [11,13,14]. However, the preoperative diagnosis of mixed tumors remains challenging. The biopsy specimen might not be sufficient for the diagnosis of pancreatic MiNEN [15]. Computed tomography (CT) and magnetic resonance imaging (MRI) are routinely used to evaluate the characteristics of focal pancreatic lesions [16][17][18], but the imaging features of pancreatic MiNEN have not been studied in detail to differentiate it from PDAC or NET. Therefore, this study was performed to investigate the CT and MRI features of pancreatic MiNEN and compare them with those of PDAC and NET.

Patient characteristics
This study was approved by our institutional review board, and patient informed consent was waived considering the retrospective nature of the study. From January 2015 to December 2020, 19 patients with pathologically confirmed pancreatic MiNEN were identified by searching medical database in our institution. Among these patients, 12 patients (male 8, female 4; mean age ± standard deviation [SD], 55.8 ± 11.1 years; range 37-73 years) were eventually enrolled in this study based on the following inclusion criteria: (1) CT and MR images included contrast enhancement (arterial phase and portal venous phase) before surgery, (2) no history of major abdominal surgery, and (3) primary pancreatic tumor. To form a 1:2 matching with the pancreatic MiNEN group, we selected 35 PDAC and 31 NET based on the above criteria. A total of 24 patients with PDAC (male 16, female 8; mean age ± SD, 58.9 ± 13.8 years; range 36-83 years) and 24 patients with pancreatic NET (male 16, female 8; mean age ± SD, 50.6 ± 14.5 years; range 31-72 years) were later selected based on gender, age, location, and treatment by using the Matchlt package of the R software (version 3.4.4, R Core Team 2017). All lesions were pathologically proven by pancreatectomy upon resection (n = 60).

CT and MRI examination CT examination
CT examinations were performed on 16 -or 64-multidetector CT scanners (Lightspeed 16 or Lightspeed 64; GE Medical Systems, Milwaukee, WI, USA) by using a dualphase scan (arterial and portal phases). Arterial and portal phase scans were obtained with a delay of 15-20 s by using a bolus-tracking technique and a delay of 60-70 s after injection of 2.5-3 mL/kg of non-ionic iodinated contrast medium intravenously (Omnipaque 300 mg/mL, GE Healthcare) at a rate of 2.5-3.5 mL/s by using a power injector. The detector collimation values of GE 16-and 64-multidetector scanners were 0.75 and 0.6 mm, respectively, the pitch was 1.5, the rotation time was 0.6 s, the tube voltage was 120 kV, the tube current was 250 mA, the reconstructed slice thickness was 4.0 mm, and the reconstruction interval was 4.0 mm.

MRI examination
MRI examinations were performed using a 1.5-T MRI scanner (GE Signa HD, GE Healthcare Systems, Milwaukee, WI, USA) with 8-and 12-channel phased-array torso coil. The applied image sequences with scan parameters were shown as follows: repetition time (TR)/echo time (TE) of 520/14 ms, section thickness of 3 mm, field of view (FOV) of 400 × 280 mm 2 , and matrix of 256 × 256 for the fast-spin-echo-based pre-contrast T1-weighted images with fat suppression; (TR/TE) of 3,500/95 ms, section thickness of 3 mm, FOV of 400 × 280 mm 2 , and matrix of 320 × 256 for the respiratory-triggered fast-spin echo-based T2-weighted images with fat suppression; TR/TE of 4.9/1.0 ms, section thickness of 3 mm, FOV of 380 × 380 mm 2 , and matrix of 320 × 288 for the 2D-fast imaging employing steady-state acquisition (2D-FIESTA) in axial and coronal views; TR/TE of 3,600/70 ms, section thickness of 5 mm, FOV of 360 × 360 mm 2 , and matrix of 128 × 128 for the free-breathing single-shot echo-planar diffusion weighted image with a and b values of 0 and 800 s/mm 2 , respectively; and TR/TE of 4.1/1.5 ms, section thickness of 3 mm, FOV of 350 × 280 mm 2 , and matrix of 320 × 256 for the contrast-enhanced T1-weighted images with contrast-enhanced phases including arterial and portal phases. These parameters were applied at 15 s (arterial phase), 60 s (portal phase) after injection of Gadodiamide intravenously (Omniscan, GE; 0.1 mmol/kg body weight) at a rate of 1.5 mL/s by using an autoinjector.

Imaging analysis
CT and MRI images were independently reviewed by two senior radiologists with 9 and 10 years of experience in abdominal imaging on the hospital image archiving and communication system. Interobserver agreement for imaging features was assessed after initial image analysis. Discrepancies between the two readers were resolved by a consensus after joint image re-evaluation. The paired CT and MRI data of 60 cases were randomly reviewed by two independent readers twice and the time interval between two assessments for each reader was least 1 month. Both readers were blinded to all clinical and pathological data.
The following lesion relevant characteristics were evaluated: largest transverse diameter of the tumor, tumor location, tumor contours and margins, heterogeneity, tumor calcification, presence of visible lymph nodes (short axis diameter > 10 mm), bile duct dilatation, presence of main pancreatic duct (MPD) enlargement (diameter > 3 mm), adjacent organ involvement, and vascular encasement by tumor. According to the internal components, tumors were classified as purely solid lesion, solid lesion with minor cystic components (cystic component < 10% of the tumor), mixed solid and cystic lesion (cystic component > 10% of the tumor) [19]. Areas with density and/or intensity similar to that of cerebrospinal fluid on both CT and MR images were considered as cystic components of tumor.
The enhancement characteristics on the arterial (pancreatic) phase include diffuse and rim type of increased enhancement. The enhancement characteristics on the portal venous phase include washout and progressive enhancement. The degree of CT enhancement was divided into no enhancement (+ 0-10 HU), mild (+ 10-20 HU), moderate (+ 20-50 HU), or marked (+ > 50 HU) enhancement compared with pre-contrast phase [20].

Clinicopathological diagnoses
Available records of clinical and pathological data were retrieved for each patient. The following clinical data were extracted from medical records: sex, age, reasons for admission (abdominal pain or discomfort or jaundice), and surgical options. Tests for the serum tumor markers, including serum carcinoembryonic antigen (CEA), serum amylase, and serum carbohydrate antigen 19-9 (CA 19-9), were available in the clinical information system. These serological exams were mainly performed within 30 days before or after CT and MRI.
All sections were retrospectively reviewed by a pathologist with 16 years of experience, who was blinded to the imaging manifestations. The pathological diagnosis was based on hematoxylin-eosin and immunohistochemical staining results.

Statistics analysis
Descriptive statistical values were calculated for all variables evaluated on CT and MRI. One-way ANOVA or Student's t test was used for continuous variables, and chi-square test or Fisher's exact test was used for categorical variables. These variables were then entered into univariate analysis with a conditional logistic regression model to determine the independent predictors of pancreatic MiNEN on CT and MRI. Multivariable logistic regression analyses were performed by adverse selection of significant variables in the univariable analyses combined with clinical significance, and intermediate factors were excluded. The sensitivity, specificity, accuracy, positive predictive value, negative predictive value (NPV), positive likelihood ratio (LR (+)), and negative likelihood ratio (LR (−)) of each significant imaging feature and combinations of these features were calculated.

Clinical features
The clinical features of 60 patients are summarized in Table 1. Significant difference was observed in serum CA19-9 levels and surgical options among the three groups (p ≤ 0.015). No significant differences were observed in any other clinicopathological findings in Table 1. All cases were confirmed by postoperative pathological diagnosis.

Unenhanced and contrast-enhanced CT and MRI imaging patterns
In CT and MRI, pancreatic MiNEN lesions were located in the head and neck (n = 4) or body and tail (n = 8) of the pancreas ( Table 1). The mean diameter of tumors was 4.7 ± 2.3 cm (range 2.5-9.0 cm), which was larger than that of PDAC (p = 0.031) and not statistically different from that of NET (p > 0.05) (   Table 2).

Diagnostic efficacy of the key imaging features
Multivariable logistic regression analyses revealed that moderate enhancement relative to PDAC (p = 0.022) and ill-defined irregular margin relative to NET (p = 0.044) were independent predictors of pancreatic MiNEN ( Table 3). The diagnostic performances on CT and MRI for these significant features and their combination are presented in Table 4. Among these parameters, moderate enhancement had the highest sensitivity (91.7%, 11/12), specificity (83.3%, 40/48), and accuracy (85%, 51/60) for the prediction of pancreatic MiNEN. In combination with tumor margin, the accuracy (91.7%, 55/60) and specificity (93.8%, 45/48) were generally higher, and the diagnosis is most likely MiNEN (LR (+) = 13.33). In addition, moderate enhancement resulted in the highest NPV (97.6%) and the lowest negative likelihood ratio (0.10).

Discussion
Our study demonstrated that pancreatic MiNEN mainly showed as a large heterogeneous mass with illdefined irregular margin, having moderate enhancement with progressive fill-in pattern. It mainly invaded adjacent organs, including spleen and colon rather than vessels. Among the parameters, ill-defined irregular margin compared with clear border of NET and moderate enhancement compared with mild enhancement of PDAC on CT and MRI were significant imaging predictors of pancreatic MiNEN. The combination of both features resulted in a high accuracy and specificity, thus Among all the features, moderate enhancement had the highest specificity (83.3%, 40/48) for predicting pancreatic MiNEN. The degree of enhancement was measured  by the dominant solid areas in the tumor due to the presence of cystic areas and heterogeneity [22]. Moderate enhancement could be clearly distinguished from PDAC (hypovascularity with progressive filling) or NET (hypervascularity with late and mild washout) [23][24][25]. Limited cases were available regarding the enhancement pattern of pancreatic MiNEN, but high-low mixed enhancement patterns have been reported, and similar results were obtained [11,26]. Moreover, the enhanced imaging features of pancreatic MiNEN can be observed as a mixture of that of PDAC and NET. Ill-defined irregular margin was a predictor of pancreatic MiNEN, which is similar to PDAC, but not same as clear border of NET [24,27,28]. Considering the high malignancy of MiNEN, the tumor may invade the boundary and extend further, which may cause illdefined irregular margins. It is a clear implication that the tumor shows infiltrative desmoplastic features which implies surgical treatment when possible. As the tumor progresses, adjacent organs and tissue can be involved. Surprisingly, vascular involvement was significantly less frequent as that of PDAC, probably because it is not as erosive as PDAC. Nießen obtained the same findings [12].
MiNEN is manifested as mixed neoplasms with both endocrine and non-endocrine differentiation. Although both adenocarcinomas and pancreatic endocrine tumors are distinct entities, their components can be combined, resulting in heterogeneous pathological and morphological appearances. Previous case reports have described the imaging appearance of MiNEN as a heterogeneous cystic and solid mass, as observed in our study [26,29].
A significant difference was observed in the largest tumor diameter between MiNEN and PDAC. Pancreatic MiNEN usually presents as a relatively large tumor [30,31]. A possible explanation for this might be that pancreatic MiNEN rarely shows obvious symptoms resulting in late detection. By comparison, PDAC usually exhibits a mean diameter of 2-3 cm [32], as observed in our study.
Neither the role of surgery nor the effect of adjuvant chemotherapy and radiotherapy is clear in pancreatic MiNEN [33]. Once it is suspected, radical resection would be preferred [12]. However, in the present study, the surgery option of pancreatic MiNEN was not completely consistent with NET or PDAC. By contrast, PDAC resection is relatively extensive and NET conservative, while MiNEN is probably somewhere in between. Moreover, it is reported that MiNEN has a better prognosis than PDAC [12]. Therefore, CT and MR imaging features could be used as a pre-surgical clinical decision support tool to help in the differential diagnosis of pancreatic MiNEN, thus potentially influencing the surgical options and chemotherapy due to the different components. The results of the present study indicate the occurrence of pancreatic MiNEN when both two imaging features (ill-defined irregular margin and moderate enhancement) are present, whereas patients without either of the two features could be considered otherwise.
Our study has some limitations. First, the small sample size might limit identification of imaging features that may be suggestive of pancreatic MiNEN, although it reflects the rarity of this tumor type. Moreover, considering the retrospective, single-center nature of the Table 4 Diagnostic performance of these significant features and combinations for predicting pancreatic MiNEN Data are number of patients, unless otherwise indicated; data in parentheses are numerator/denominator of patients; data in square brackets are 95% CIs MiNEN, mixed neuroendocrine-non-neuroendocrine neoplasm; PPV, positive predictive value; NPV, negative predictive value; LR (+), positive likelihood ratio; LR (−), negative likelihood ratio

Imaging findings
Tumor composition study with an inherent selection bias as we only evaluated the resected tumors, our findings may not represent the full spectrum of pancreatic MiNEN.
In conclusion, we have mainly described the radiological findings of pancreatic MiNEN with ill-defined irregular margin and moderate enhancement compared with PDAC and NET. The combination of imaging features could improve the diagnostic efficiency and help in the selection of appropriate treatment.